This invention concerns epoxy resin mixtures for producing prepregs and composites and it also concerns the prepregs and composites produced from said epoxy resin mixtures.
Composites based on epoxy resins and organic or inorganic reinforcing materials have gained an important position in many areas of the industry and daily life. The reasons include first the relatively simple and reliable processing of epoxy resins and secondly the good mechanical and chemical properties of cured epoxy resin molding materials that make it possible to adapt to a variety of applications and permit advantageous utilization of the properties of all the materials that make up the composite.
Epoxy resins are preferably processed to composites by the intermediate step of producing prepregs first. For this purpose, organic or inorganic reinforcing materials or embedding components in the form of fibers, nonwovens and woven fabrics as well as other flat materials are impregnated with the resin. In most cases, this is accomplished with a solution of the resin in a volatile solvent. The resulting prepregs according to this process must not be sticky but they should not be fully cured either. Instead, the resin matrix should simply be prepolymerized. In addition, the prepregs must have sufficient storage stability. Thus, for example, a storage stability of at least three months is required for the production of circuit boards. In further processing to form composites, the prepregs must also melt at higher temperatures and they must form a strong and permanent bond under pressure with both the reinforcing materials or embedding materials and the materials intended for the composite--in other words, the cross-linked epoxy resin material must yield a high interlaminas adhesion with the reinforcing materials or the embedding components and with the materials to be bonded, such as metallic, ceramic, mineral and organic materials.
In general, a high mechanical and thermal strength, good mechanical and thermal stability, thermal undeformability, and good aging resistance are required of composites when cured. For electrotechnical and electronic applications, there is also the requirement that the electric insulation properties must be permanently high, and for many special applications there are numerous additional requirements. For example, good thermal undeformability over a wide temperature range, good adhesion to glass and copper, a high surface resistance, a low dielectric loss factor, good machining properties (punchability, drillability), a low water uptake and high corrosion resistance are required if these materials are to be used as circuit board materials.
With increasing loads and extensive use of the composites, the thermal undeformability requirement becomes especially important. This means that the materials must withstand high temperatures during processing and use without showing any deformation or damage to the composite--due to delamination, for example. In dip soldering, for example, circuit boards are exposed to a temperature of 270.degree. C. Likewise, local temperatures of more than 200.degree. C. may occur briefly during cutting and drilling operations. Materials with a high glass transition temperature T.sub.G have good properties in this regard. If the glass transition temperature is higher than these temperatures, thermal undeformability is assured in the entire temperature range covered during processing, and damage such as warpage and delamination can be largely ruled out. The epoxy resin currently used on a large scale worldwide for FR4 composites has a glass transition temperature of only 130.degree. C. after curing. However, this results in the damage described above and thus leads to rejects in production. Therefore, there has long been a demand for inexpensive materials that can be processed relatively well and have a glass transition temperature higher than about 180.degree. C.
Another requirement that has become increasingly important in recent times is that these materials must be flame retardant. In many areas, this requirement is a top priority because of the danger to humans and property--for example, in construction materials for aircraft and motor vehicles and for public transportation. Flame retardancy of circuit board materials is an indispensable requirement for electrotechnical applications and especially for electronic applications because of the high value of the electronic components mounted on the circuit boards.
Therefore, in order to evaluate flammability, a material must pass one of the most severe material test standards, namely the V-0 classification according to UL 94V. In this test, a test article is exposed to a deformed flame vertically at the lower edge. The total burning times in 10 tests must not exceed 50 sec. This requirement is difficult to satisfy, especially when the material has thin walls, as is customary in electronics. The epoxy resin used industrially throughout the world for FR4 composites meets this requirement only because it contains approx. 30-40% (based on the resin) ring-brominated aromatic epoxy components--in other words, it contains approx. 17-21% bromine. Comparably high concentrations of halogen compounds are used for other applications and are often combined with antimony trioxide as a synergist. The problem with these compounds is that although they are excellent flame retardants, they also have some highly objectionable properties. For example, antimony trioxide is on the list of carcinogenic chemicals, and aromatic bromine compounds not only release highly corrosive free bromine radicals and hydrogen bromide when they undergo thermal decomposition, but especially when decomposition occurs in the presence of oxygen, highly brominated aromatics can also lead to polybromo-dibenzofurans and polybromo-dibenzodioxins (PBDs) which are extremely toxic. Furthermore, disposal of waste materials containing bromine also poses considerable problems.
Materials that meet the demand for a good thermal stability include, for example, molding materials based on bismaleinimid/triazine (BT) with a T.sub.G of approx. 200.degree. C. or polyimide (PI) with a T.sub.G of 260.degree.-270.degree. C. Recently, BT-epoxy blends with a T.sub.G of 180.degree. C. have also been, available, but laminates produced with these resin systems cannot be processed or machined as well as laminates based on epoxy resin. Thus, for example, production of laminates based on polyimide requires a compression molding temperature of approx. 230.degree. C. and a much longer post-curing time (approx. 8 hrs) at temperatures of 230.degree. C. Another serious disadvantage of these resin systems is that the price of materials is 6 to 10 times higher.
A comparatively inexpensive resin system is obtained when aromatic and/or heterocyclic polyepoxy resins, i.e., polyglycidyl compounds, are combined with aromatic polyamines acting as the hardener. Such polyamines, which are disclosed in German patent 2,743,680, for example, lead to cross-linked polymers that have an especially good thermal undeformability under heat and good resistance to aging. According to European patent 274,646, when 1,3,5-tris-(3-amino-4-alkylphenyl)-2,4,6-trioxo-hexahydrotriazines are used as hardeners, the resulting laminates have a glass transition temperature of up to 245.degree. C. and are characterized by good processing and machining properties.
Although the flammability properties of these resin systems may vary greatly, they all have the disadvantage that their flame retardancy is inherently inadequate. Thus, in order to pass the flammability test according to UL 94V with a classification of V-0, which is essential for many applications, it has not yet been possible to do without the use of the highly effective brominated flame retardants. First, this results in the potential hazards associated with bromine compounds, and secondly, there are some unavoidable negative effects on the thermal and mechanical properties due to the use of bromine compounds.
For these reasons, there has been no lack of attempts to replace brominated flame retardants with less problematical substances. For example, fillers with a quenching gas effect have been proposed, such as aluminum oxide hydrates (see: J. Fire and Flammability, vol. 3 (1972), pages 51 ff.), basic aluminum carbonates (see: Plast. Engng., vol. 32 (1976), pages 41 ff.) and magnesium hydroxides (European patent 243,201) as well as vitrifying fillers such as borates (see: Modern Plastics, vol. 47 (1970), no. 6, pages 140 ff.) and phosphates (U.S. Pat. Nos. 2,766,139 and 3,398,019). However, all these fillers have the disadvantage that they often have an extremely negative effect on the mechanical, chemical and electrical properties of the composites. In addition, they require special processing techniques which are usually more expensive because they increase the viscosity of the filled resin system and they also have a tendency toward sedimentation.
The flame-retardant effect of red phosphorus has also been described previously (British-patent 1,112,139), optionally in combination with very finely divided silicon dioxide or aluminum oxide hydrate (U.S. Pat. No. 3,373,135). This yields materials that restrict the possible uses for electrotechnical and electronic applications because of the phosphoric acid that is released in the presence of moisture and the resulting corrosion problems. In addition, organic phosphorus compounds such as phosphoric acid esters, phosphonic acid esters and phosphines have already been proposed as flame-retardant additives (see: W. C. Kuryla and A. J. Papa Flame Retardancy of Polymeric Materials, vol. 1, pages 24-38 and 52-61, Marcel Dekker Inc., New York, 1973). Since these compounds are known for their "plasticizing" properties and are in fact used internationally as plasticizers for polymers on a large scale (British patent 10,794), this alternative is not very promising either.
Organic phosphorus compounds such as phosphorus compounds that contain epoxy groups and can be anchored in the epoxy resin network can also be used to make epoxy resins flame-retardant. Thus, for example, European patent 384,940 discloses epoxy resin blends that contain commercially available epoxy resin, the above-mentioned aromatic polyamine 1,3,5-tris-(3-amino-4-alkylphenyl)-2,4,6-trioxo-hexahydrotriazine and a phosphorus compound that contains an epoxy group and is based on glycidyl phosphate, glycidyl phosphonate or glycidyl phosphinate. Without adding halogen, flame-retardant laminates or composites that can be classified in the UL 94 V-0 class and have a glass transition temperature of &gt;200.degree. C. can be produced with such epoxy resin blends. In addition, these epoxy resin blends can be processed by methods comparable to those used with the current epoxy resins.
It is general knowledge that the interlaminar adhesion and the adhesion to copper of laminates with a high glass transition temperature, such as those based on polyimide or BT resins, is lower than that of the halogenated FR4 laminates used predominantly today. This is also true of the laminates described in European patent 384,940. Many of the circuit boards produced today are so-called multi-layer circuit boards (ML) that have several conducting layers that are kept at a distance from each other and are insulated by epoxy resin compounds. However, the trend in ML technology has been toward an ever increasing number of conducting layers. For example, today MLs with more than 20 conducting levels are being produced. An excessive thickness of the ML must be avoided for technical reasons, so the distance between the conducting levels becomes smaller and smaller and thus the interlaminar adhesion and the adhesion to copper become increasingly problematical with ML laminates that have a high glass transition temperature.
In circuit board technology, interlaminar adhesion is usually determined indirectly. Circuit boards must pass a measling test that is widely used for this purpose. In this test, a laminate without the copper lamination is treated with a tin chloride solution and then with water at a high temperature and then is immersed for 20 sec in a hot soldering bath at 260.degree. C. The laminate is then inspected visually for delamination. With regard to lamination resins with a high glass transition temperature (180.degree. C. or higher), most of the increasingly thin core components used in ML technology today will not pass this test because their interlaminar adhesion is inadequate for such thin laminates. Other problems caused by inadequate interlaminar adhesion are encountered in further processing of laminates for the electronics industry by such methods as drilling and milling. Therefore, the drilling and milling speeds must be reduced in comparison with those used with FR4 material.
Therefore, there has been a great demand for electronics laminates that first of all meet the required level of flame retardancy without using halogen, as indicated above, and secondly have a high glass transition temperature with good interlaminar adhesion at the same time--even when extremely thin core components are produced. No satisfactory combination of these properties has previously been achieved, especially not for extremely thin laminates such as those use in ML technology.